On Sat, 31 Dec 2016 02:14 am, Serhiy Storchaka wrote: > On 30.12.16 16:47, Steve D'Aprano wrote: >> Again, assume both operands are in range for an N-bit signed integer. >> What's a good way to efficiently, or at least not too inefficiently, do >> the calculations in Python? > > def to_unsigned(bits, x): > return x & ((1<<bits)-1) > > def to_signed(bits, x): > offset = 1<<(bits-1) > return to_unsigned(bits, x + offset) - offset
Thanks. Are you saying that the best way of doing this is this? (1) convert signed Python ints to unsigned; (2) perform operation and bitmask; (3) convert unsigned back to signed. Here's an example. I want to multiply 7*3 using a signed 4-bit int, getting 5 as the answer, and 7*4 getting -4 as the answer: py> N = 4 py> to_signed(N, to_unsigned(N, 7) * to_unsigned(N, 3) & (2**N - 1)) 5 py> to_signed(N, to_unsigned(N, 7) * to_unsigned(N, 4) & (2**N - 1)) -4 It works, but I'm glad I won't be doing anything that requires great speed :-) -- Steve “Cheer up,” they said, “things could be worse.” So I cheered up, and sure enough, things got worse. -- https://mail.python.org/mailman/listinfo/python-list
